The invention relates to cutting elements for rotary drill bits for use in drilling or coring holes in subsurface formations.
In particular, the invention is applicable to cutting elements for use on rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string and an inner passage for supplying drilling fluid to the face of the bit, the bit body carrying a plurality of cutting elements. Each cutting element comprises a preform element, often in the form of a circular tablet, including a cutting table of superhard material having a front cutting face and a rear face, the rear face of the cutting table being bonded to a substrate of material which is less hard than the superhard material.
The cutting table, which is normally in the form of a single layer, usually comprises polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride. The substrate of less hard material is often formed from cemented tungsten carbide, and the cutting table and substrate are bonded together during formation of the cutting element in a high pressure, high temperature forming press.
Since the substrate is of less hard material than the cutting table, the two-part arrangement of the cutting element provides a degree of self-sharpening since, in use, the less hard substrate wears away more easily than the harder cutting table.
The perform cutting element may be directly mounted on the bit body or may be bonded to a carrier, for example also of cemented tungsten carbide, the carrier being in turn received in a socket in the bit body. The bit body may be machined from metal, usually steel, or may be formed from an infiltrated tungsten carbide matrix by a powder metallurgy process.
Such cutting elements are subjected to extremes of temperature and heavy loads, including impact loads, when the drill is in use down a borehole. It is found that under drilling conditions spalling of the diamond table can occur, that is to say the separation and loss of diamond material over the cutting surface of the table. Such spalling usually spreads from the cutting edge, probably as a result of impact forces. The spalling reduces the cutting efficiency of the element, and in severe cases can lead to delamination, that is to say separation of the diamond table from the substrate.
It has been found that the incidence of spalling may be reduced by pre-bevelling the periphery of the diamond table. Reference in this regard may be made to U.S. Pat. No. Re. 32036. However, the pre-bevelling process is comparatively costly and time consuming, involving as it does the cutting or grinding of a significant amount of the superhard material from the periphery of the diamond table. Also, the pre-formed bevel becomes worn away after an initial period of use and the anti-spalling advantage it gives therefore also disappears after such period.
The present invention sets out to provide an alternative and advantageous method of forming the cutting table of superhard material in a cutting element in a manner to inhibit spalling.